Myrcella recast would suck. Recasts suck in general and that girl was so into the show lol. Maybe she sucks at acting though? Who knows. That said, it is just a tumblr post, and given that they are casting young Cersei, it seems like it'd be really easy for the wires to get crossed in the rumor mill and have people thinking that the casting for young Cersei is actually a new Myrcella.
As for Stoneheart, things that appear in the episode aren't necessarily shown in the recap. Last year episode 10 had that scene at Pyke with Balon and Yara getting the dick in a box, but neither characters were shown in the recap. It does seem like they would've at least mentioned the Brotherhood or shown Sansa's clip where she talks about them dumping her mother in the river.
If there's no Stoneheart though, and we're definitely getting a Brienne/Hound encounter this week, then I have no idea where they're going with Brienne for next season. Just her and Pod wandering aimlessly after failing to get Arya? This whole season for her seemed carefully constructed to avoid exactly that, since people never really liked the meandering nature of her plot in the books. Her plot arc really doesn't make much sense without ending on Stoneheart.